1. Field of the Invention
The present invention generally relates to control systems for regulating the flow of fluids. More specifically, this invention relates to a gas control system for regulating the flow of a gas into a mold during an injection molding process, wherein the quantity of gas injected is accurately controlled in a manner that permits statistical monitoring and evaluation of the molding process.
2. DESCRIPTION OF THE PRIOR ART
Within the plastic molding industry, it is well known to inject a pressurized gas along with a quantity of molten plastic, or melt, into the mold cavity during the molding process. Because the pressurized gas will flow along a path of least resistance, within the mold cavity the gas flows through the warmer, less viscous melt which is located away from the mold cavity surfaces. As a result, the gas generally flows through the approximate center of the flow path cross section, urging the melt into contact with the surfaces of the mold cavity. At the same time, the gas forms a hollow channel or void within the molded article during the molding process.
The use of gas serves several useful purposes. By continuously applying pressure throughout the molding cycle, the gas maintains the melt in contact with the mold cavity so as to force the melt to assume the shape of the mold cavity and thereby reduce the tendency for surface flaws, sink holes and warpage. To achieve this aspect, the gas must remain pressurized until the melt has sufficiently cooled within the mold cavity. Another benefit is that the gas assists in forming long and narrow molded articles in that the gas rapidly urges the melt through the mold cavity, preventing the melt from prematurely solidifying prior to completely filling the mold cavity. Again, because the gas will tend to flow through the least viscous melt at the approximate center of the mold cavity, the melt will cool and solidify from the surface inward towards the center of the mold cavity. This effect yields another advantage in that the resulting hollow structure of the molded articles will, by nature, be lighter, using less plastic material for a given geometry and size.
Numerous gas-assisted molding methods and systems have been suggested by the prior art. Examples include U.S. Pat. Nos. 5,039,463 to Loren and 5,056,997 to Hayashi et al., each of which control the injection of the gas from the standpoint of controlling the gas pressure. Specifically, both Loren and Hayashi et al. attempt to perform the gas-assisted injection process by introducing and temporarily holding the gas at one or more predetermined gas pressure levels, or "hold" pressures, within the mold cavity until the plastic has sufficiently solidified.
Though both Loren and Hayashi et al. each begin with a "fixed" volume of gas, the references teach that it is specifically the pressure which is regulated at a predetermined level during the injection process in order to accomplish the invention set forth therein. In essence, the volume of gas used by both Loren and Hayashi et al. varies during the injection process in that the hold pressure is regulated from a higher pressure, necessitating that some gas will be vented from the system to attain the preferred, lower hold pressure. For example, Loren teaches charging a receiver with a gas to an extremely high fixed pressure (14,000 psi), and then regulating the gas to a desired pressure level (6000 psi) before introducing it into the mold cavity. The desired pressure level can be sustained throughout the mold cycle due to the reservoir of gas available from the receiver.
Alternatively, Loren teaches that a fixed volume storage system can be charged to 6000 psi by the receiver, after which the fixed volume storage system is shut off from the receiver. The 6000 psi gas within the fixed volume storage system can then be vented to the mold cavity, such that the mold cavity is initially pressurized to 6000 psi with the gas. The pressure is then gradually reduced to about 1000 psi by a regulating relief valve. The 1000 psi pressure is then held for the remainder of the cycle, during which the plastic melt solidifies.
While controlling the gas-assisted injection process by regulating the pressure generally works satisfactorily, a significant disadvantage with such an approach is that there is no means provided by which an operator can determine whether the gas has actually reached or entered the mold cavity. As with the teachings of Loren, the gas pressure is both regulated and detected well upstream of the mold cavity. As a result, the gas supply line can unknowingly be obstructed downstream from the regulator or pressure sensor by such things as contaminants in the gas or by plastic within the sprue or gates which feed both the gas and plastic melt to the mold cavity. In a high capacity production process such as injection molding, parts resulting from the above defective process will typically go unnoticed until many parts have been produced, resulting in a substantial loss of time and a high scrap rate.
It is obviously impractical to regulate the gas pressure downstream of the mold cavity during the injection cycle in an attempt to overcome the above shortcoming. While it is possible to provide a pressure sensor in direct communication with the mold cavity, such an approach is generally unacceptable from the standpoint of surface blemishes and defects created on the molded article. Even if attempted, the pressure sensor may likely relay a faulty reading due to being in direct contact with the liquid plastic and not the pressurized gas. Furthermore, such a pressure sensor would be exposed to cyclical high temperatures, which would have an adverse effect on the accuracy and life of the sensor.
From the above discussion, it can be readily appreciated that the prior art does not disclose a gas-assisted injection system which is capable of immediately detecting when the gas has failed to enter and expand the melt within the mold cavity. Consequently, the prior art is unable to immediately notify a system operator or cause a system shutdown upon the occurrence of such a failure.
Accordingly, what is needed is an uncomplicated system for injecting gas into a mold cavity wherein the system is able to immediately and continuously detect whether the gas has in fact entered the mold cavity and, as a result, whether the injection molding process is producing flawed parts from the standpoint of inadequate gas fill or pressure during the molding operation.